Hematopoiesis is a paradigmatic system for progressive restriction of cell fate potentials from hematopoietic stem cells to lineage-restricted progenitors that produce all lineages of mature blood cells. Transcription factors play a critical role in hematopoietic development through the activation or silencing of a number of differentiation-related genes, and are likely to be the ultimate effectors for hematopoietic lineage decision. Recent studies show that the functional alterations of transcription factors are linked to the development of acute leukemia. Thus, our focus of this study is to understand the physiological regulation of transcription factors on hematopoiesis, as well as to understand the effect of their deregulation on hematopoietic homeostasis. We recently identified multiple myeloid and lymphoid-restricted progenitors downstream of hematopoietic stem cells, which enabled us to directly evaluate the distribution of transcription factor at each branching point of hematopoiesis. To further understand the stage-specific effects of transcription factors, we will establish a mouse line having transgenic reporters for GATA-1 and PU.1, purify cells with different endogenous GATA-1/PU.1 profiles, and test lineage potentials of cells with each GATA-1/PU.1 profile. We will then enforce the expression of GATA-1 and PU.1 as well as CEBPalpha in lineage-restricted progenitor or stem cells to test whether enforced transcription factors can specify or change their cell fates. Finally, we will evaluate the effect of loss-of-function of CEBPalpha on granulocyte/monocyte lineage decision by using a CEBPalpha knockout model. All of these methods will be combined with our progenitor isolation system to precisely analyze their roles at each stage of hematopoiesis. Thus, we hope these studies will help understand mechanistic effects of these transcription factors on lineage decision in normal and malignant hematopoietic development.